riscv-atomic.adoc

RISC-V Atomics ABI Specification

RISC-V atomics mappings

This specifies mappings of C and C++ atomic operations to RISC-V machine instructions. Other languages, for example Java, provide similar facilities that should be implemented in a consistent manner, usually by applying the mapping for the corresponding C++ primitive.

Note

Because different programming languages may be used within the same process, these mappings must be compatible across programming languages. For example, Java programmers expect memory ordering guarantees to be enforced even if some of the actual memory accesses are performed by a library written in C.

Note	Though many mappings are possible, not all of them will interoperate correctly. In particular, many mapping combinations will not correctly enforce ordering  between a C++ memory_order_seq_cst store and a subsequent memory_order_seq_cst load.

Note

These mappings are very similar to those that originally appeared in the appendix of the RISC-V "unprivileged" architecture specification as "Mappings from C/C++ primitives to RISC-V Primitives", which we will refer to by their 2019 historical label of "Table A.6". That mapping may be used, except that atomic_store(memory_order_seq_cst) must have an an extra trailing fence for compatibility with the "Hypothetical mappings …​" table in the same section, which we similarly refer to as "Table A.7". As a result, we allow the "Table A.7" mappings as well.

Note

Our primary design goal is to maximize performance of the "Table A.7" mappings. These require additional load-acquire and store-release instructions, and are this not immediately usable. By requiring the extra store fence. or equivalent, we avoid an ABI break when moving to the "Table A.7" mappings in the future, in return for a small performance penalty in the short term.

For each construct, we provide a mapping that assumes only the A extension. In some cases, we provide additional mappings that assume a future load-acquire and store-release extension, as denoted by note 1 in the table.

All mappings interoperate correctly, and with the original "Table A.6" mappings, except that mappings marked with note 3 do not interoperate with the original "Table A.6" mappings.

We present the mappings as a table in 3 sections. The first deals with translations for loads, stores, and fences. The next two sections address mappings for read-modify-write operations like fetch_add, and exchange. The second section deals with operations that have direct amo instruction equivalents in the RISC-V A extension. The final section deals with other read-modify-write operations that require the lr and sc instructions.

Table 1. Mappings from C/C++ primitives to RISC-V primitives

C/C++ Construct	RVWMO Mapping	Notes
Non-atomic load	l{b|h|w|d}
atomic_load(memory_order_relaxed)	l{b|h|w|d}
atomic_load(memory_order_acquire)	l{b|h|w|d}; fence r,rw
atomic_load(memory_order_acquire)	<RCsc atomic load-acquire>	1, 2
atomic_load(memory_order_seq_cst)	fence rw,rw; l{b|h|w|d}; fence r,rw
atomic_load(memory_order_seq_cst)	<RCsc atomic load-acquire>	1, 3
Non-atomic store	s{b|h|w|d}
atomic_store(memory_order_relaxed)	s{b|h|w|d}
atomic_store(memory_order_release)	fence rw,w; s{b|h|w|d}
atomic_store(memory_order_release)	<RCsc atomic store-release>	1, 2
atomic_store(memory_order_seq_cst)	fence rw,w; s{b|h|w|d}; fence rw,rw;
atomic_store(memory_order_seq_cst)	amoswap.rl{w|d};	4
atomic_store(memory_order_seq_cst)	<RCsc atomic store-release>	1
atomic_thread_fence(memory_order_acquire)	fence r,rw
atomic_thread_fence(memory_order_release)	fence rw,w
atomic_thread_fence(memory_order_acq_rel)	fence.tso
atomic_thread_fence(memory_order_seq_cst)	fence rw,rw

C/C++ Construct	RVWMO AMO Mapping	Notes
atomic_<op>(memory_order_relaxed)	amo<op>.{w|d}	4
atomic_<op>(memory_order_acquire)	amo<op>.{w|d}.aq	4
atomic_<op>(memory_order_release)	amo<op>.{w|d}.rl	4
atomic_<op>(memory_order_acq_rel)	amo<op>.{w|d}.aqrl	4
atomic_<op>(memory_order_seq_cst)	amo<op>.{w|d}.aqrl	4

C/C++ Construct	RVWMO LR/SC Mapping	Notes
atomic_<op>(memory_order_relaxed)	loop:lr.{w|d}; <op>; sc.{w|d}; bnez loop	4
atomic_<op>(memory_order_acquire)	loop:lr.{w|d}.aq; <op>; sc.{w|d}; bnez loop	4
atomic_<op>(memory_order_release)	loop:lr.{w|d}; <op>; sc.{w|d}.rl; bnez loop	4
atomic_<op>(memory_order_acq_rel)	loop:lr.{w|d}.aq; <op>; sc.{w|d}.rl; bnez loop	4
atomic_<op>(memory_order_seq_cst)	loop:lr.{w|d}.aqrl; <op>; sc.{w|d}.rl; bnez loop	4
atomic_<op>(memory_order_seq_cst)	loop:lr.{w|d}.aq; <op>; sc.{w|d}.rl; bnez loop	3, 4

Meaning of notes in table

1) Depends on a load instruction with an RCsc aquire annotation, or a store instruction with an RCsc release annotation. These are curently under discussion, but the specification has not yet been approved.

2) An RCpc load or store would also suffice, if it were to be introduced in the future.

3) Incompatible with the original "Table A.6" mapping. Do not combine these mappings with code generated by a compiler using those older mappings. (This was mostly used by the initial LLVM implementations for RISC-V.)

4) Currently only directly possible for 32- and 64-bit operands.

Other conventions

It is expected that the RVWMO AMO Mappings will be used for atomic read-modify-write operations that are directly supported by corresponding AMO instructions, and that LR/SC mappings will be used for the remainder, currently including compare-exchange operations. Compare-exchange LR/SC sequences on the containing 32-bit word should be used for shorter operands. Thus, a fetch_add operation on a 16-bit quantity would use a 32-bit LR/SC sequence.

It is acceptable, but usually undesirable for performance reasons, to use LR/SC mappings where an AMO mapping would suffice.

Atomics do not imply any ordering for IO operations. IO operations should include sufficient fences to prevent them from being visibly reordered with atomic operations.

Float and double atomic loads and stores should be implemented using the integer sequences.

Float and double read-modify-write instructions should consist of a loop performing an initial plain load of the value, followed by the floating point computation, followed by an integer compare-and-swap sequence to try to store back the updated value. This avoids floating point instructions between LR and SC instructions. Depending on language requirements, it may be necessary to save and restore floating-point exception flags in the case of an operation that is later redone due to a failed SC operation.

Note

The "Eventual Success of Store-Conditional Instructions" section in the ISA specification provides that essential progress guarantee only if there are no floating point instructions between the LR and matching SC instruction. By compiling such sequences with an "extra" ordinary load, and performing the floating point computation before the LR, we preserve the guarantee.